If you like challenging yourself with small strategy games, you found it in Crystal Defenders.
It has your basic class system found in most tower defense games, your Warrior that does heavy but ground only attacks, archers which to medium but air-only attacks, Mages that shoot their spells causing splash damage. And to add some complexity to make you think a little more, Crystal Defenders adds "mix" classes that mix the classic classes as towers so you can truly customize how to defend your life.
Crystal Defenders doesn't provide you with anything that can't find anywhere online for free and 100 times better. But if you like simple tower defense games with normal upgrade features and a handful of levels then its not bad. But truly, save your MS Points or money and go play a better one online.